This invention relates to a signal processing device in a video printer in which a thermal head is used for the hard-copying of TV (television) broadcast images or the like.
Recently, a video printer for readily hard-copying video images has been developed, and a number of patent applications have been filed with the Patent Office. FIG. 24 shows one example of a conventional video printer signal processing device which has been disclosed by Japanese Patent Application (OPI) No. 84671/1987 (the term "OPI" as used herein means an "unexamined published application"). In the signal processing device, printing is carried out with a line divided into a plurality of blocks for the purpose of economical use of electric power.
In FIG. 24, reference numeral 101 designates analog signal processing means; 102, an analog-to-digital (A/D) converter; 103, an image memory; 104, memory control means; 105, a digital-to-analog (D/A) converter; 106, analog signal output processing means; 107, color selecting means; 108, a line memory; 150, joint processing means comprising a data read-only memory 109 (hereinafter referred to as "a ROM 109", when applicable) and a correction data memory 110; and 111, printing control means.
Further in FIG. 24, reference numeral 160 designates a half-tone control means comprising a correction data inserting circuit 112, a white data inserting circuit 113, a white data generating circuit 114, a data processing circuit 115, and gradation pulse generating means 116; 117, a thermal line head; 118, a temperature sensor; and 119, temperature signal conversion means.
The operation of the video printer signal processing device thus organized will be described.
An image signal applied to the video input terminal 170 is supplied to the analog signal processing means 101, where it is converted into R (red), G (green) and B (blue) color signals. The color signals are applied to the analog-to-digital converter circuit 102, where they are converted into R, G and B digital color signals. The R, G and B digital color signals are simultaneously stored by the image memory 103. The three-color digital image data stored in the image memory 103 are read out at the same speed by the memory control means 104 as they are stored. Furthermore, these digital image data are converted by the digital-to-analog converter 105 into the three-color (R, G and B) analog signals which are the same as those of the memory input images. The analog signals are supplied to the analog signal output processing means 106, where they are converted into a video signal.
On the other hand, one of the R, G and B digital signals; that is, one color is selected by the color selecting means 107 and stored in the line memory 108. The term "one line of data" as used herein is intended to mean "a vertical line of data" as shown in FIG. 25. One line of data are not printed at a time; that is, they are divided into a plurality of blocks (two blocks in this case) for printing. Of these data, the data at the joints of blocks is supplied to the data ROM 109, where it is subjected to correction. The data thus processed is applied to the corrected data memory 110. In the conversion of data, the output temperature signal of the temperature sensor 118 is applied through the temperature signal conversion means 119, as a digital signal, to the data ROM 109, so that conversion data corresponding to the head temperature at that time instant is outputted with the digital signal as address.
One line of image data stored in the line memory 108 is supplied to the half-tone control means 160. In the half-tone control means 160, the data at the joint of blocks is replaced by joint correction data by the correction data inserting circuit 112, and is converted by the white data inserting circuit 113 into data trains suitable for division printing as shown in the parts (c) and (b) of FIG. 26, which are supplied through the data processing circuit 115 to the thermal line head 117, so that they are printed. The time interval that the thermal line head is electrically energized is determined by the output strobe pulse of the gradation pulse generating means 116.
As was described above, when the printer accomplishes the transfer of data for each block, the next line (which is the line on the right of the thermal line head in FIG. 25) of data is loaded in the line memory 108, the printing operation is started again.
In the video printer, its thermal head is arranged vertically of the TV picture as shown in FIG. 25. The printing is carried out line by line from the left most line to the rightmost line as indicated by the arrow in FIG. 25. That is, the printing of one color is ended at the rightmost line. The printer employs the system of printing colors successively which is generally employed by thermal. That is, when a printing mechanism (not shown) accomplishes a printing operation in one color, the printed sheet is set at the original printing start position, and a printing operation in the next color is carried out. When the printing operations in three colors are carried out in this manner, the printing operation of one sheet is ended.
A one line division printing operation will be described with reference to FIG. 26. In this connection, it is assumed that the thermal head has 512 heat-generating resistors, the 1st through 256-th heat resistors for a block A, and the 257-th through 512-th heat generating resistors for a block B.
In the division printing operation, in the first process the block B is printed, while the block A is not printed (as shown in the part (b) of FIG. 26); and in the next process, the block A is printed, but the block B is not printed (as shown in the part (a) of FIG. 26). Thus, the printing of one line is accomplished by performing the block printing operation twice. In FIG. 26, Dn represents the data of the n-th heating generating element, and .alpha. represents the correction rate of the joint of blocks.
The reason why correction is required for the joint of blocks is as follows: In the thermal type printer in which the printing operation is carried out with one line divided into two blocks, the heat generating resistor at the middle of the thermal head is cooled by the block which is not heated, as a result of which the printing density is lowered. Accordingly, if no correction is given to the joint of blocks, then the boundary between the blocks is lowered in printing density when printed, thus showing a white stripe. In order to eliminate this difficulty, the correction of printing density is carried out for the joint of blocks (hereinafter referred to as "joint density correction", when applicable).
Thus, in the division printing method, as for each line, the joint of blocks is printed twice, and the remaining parts are printed only once.
Now, the necessity of changing the correction data for the joint of blocks according to temperature will be described. FIG. 27 is a graphical representation indicating gradation with electrical energization time of the head.
In FIG. 27, the curve A indicates normal data with a temperature a; the curve B, normal data with a temperature b; the curve C, correction data with the temperature a; and the curve D, correction data with the temperature b, where a&lt;b.
In order to print with a gradation m, the electrical energization time of the head should be t.sub.A with the temperature a, and t.sub.B with the temperature b; and for the joint portion, the electrical energization time should be t.sub.C .times.2 with the temperature a, and t.sub.D .times.2 with the temperature b, where t.sub.A /t.sub.B =t.sub.C /t.sub.D. Thus, the correction data must be changed with temperature.
In this case, the correction data can be changed with temperature and with gradation.
This joint density correction is achieved when the joint processing means 150 corrects the input data. That is, the joint portion's data to be supplied to the head is subjected to data correction by the data ROM 109.
A plurality of trains of correction data according to head temperatures have been written in the data ROM 109 in advance.
The head 117 has the temperature sensor 118, as was described above. A head temperature detected by the temperature sensor 118 is converted into a digital temperature signal by the temperature signal conversion means 119. With the aid of the digital temperature signal, the data ROM 109 switches the above-described correction data groups, and supplies the correction data assigned to the temperature to the correction data memory 110, where it is stored.
Thereafter, according to the above-described printing operation, data is transferred from the line memory 108 to the half-tone control means 160. In this operation, the timing of data to be corrected being detected, the armature of the correction data inserting circuit 112 is operated to supply the output correction data of the correction data memory 110 to the data processing circuit 115. In this connection, the white data inserting circuit 113 has been so operated that the data transferred to the not-printed block of the head is replaced by white data.
One example of the arrangement of the joint processing means 150 and the half-tone control means 160 will be described with reference to FIG. 28, in which parts corresponding functionally to those which have been described with reference to FIG. 24 are therefore designated by the same reference numerals or characters.
In FIG. 28, reference numeral 218 designates a data comparator; 219, a data discriminator; 220, a decoder; 221, a gamma read-only memory (hereinafter referred to merely as "a ROM", when applicable); 222, a temperature characteristic correction data selector; 118, a temperature sensor; 224, a temperature signal amplifier; 225, an analog-to-digital (A/D) converter; 226, a microcomputer; and 227, gradation control means.
The operation of the circuit shown in FIG. 28 will be described. In this connection, it is assumed that one line of data is stored in the line memory 108. First, the print control means 111 reads from the line memory 108 data groups with head addresses to be corrected, and supplies them to the data ROM 109. The data ROM 109 outputs correction data corresponding to the input data. In the correction data memory 110, the write/read address is determined by the decoder 220. Thereafter, the printer is placed in printing state, and the data from the line memory 108 are successively supplied through the correction data inserting circuit 112 to the following stage.
When, in this case, the transfer timing of data to be corrected occurs, the armature of the correction data inserting circuit 112 is tripped by the printing control section 111 to select one of the inputs, that is, to select the correction data supplied by the correction data memory 110. These continuous data trains are supplied through the white data inserting circuit 113 to the data comparator 218. In the data comparator 218, the input data is compared with gradation data provided by the gradation control means 227, so that electrical energization data for controlling the electrical energization of (turning on and off) the heat-generating element on the thermal head 117 is outputted. The electrical energization data is applied to one of the blocks of the head by means of the data discriminator 219.
After the transfer of data to the thermal head 117, the gradation pulse generating means 116 outputs a strobe pulse to permit the electrical energization of the heat-generating elements. Thereafter, the line memory 108 outputs data, which is applied through the correction data inserting circuit 112, the white data inserting circuit 113, and the data processing circuit 115 to the thermal head 117, to permit the electrical energization of the heat-generating elements. In this case, as shown in the parts (a) and (b) of FIG. 26, by means of the data discriminator 219, the white data to the head is distributed to the block which is opposite to that in the previous data arrangement. Thus, in the case of the two-block head, the above-described transfer of data is carried out twice, the printing of one line is achieved.
Now, a temperature control system for the joint processing means will be described.
At the beginning of the transfer of a line, the line memory 108 applies the data on the joint portion thereof to the data ROMs 109 which are different in data content separated according to temperatures. On the other hand, the temperature sensor 118 is provided near the joint of blocks of the thermal head 117, outputting a temperature signal at all times. The temperature signal is amplified by the temperature signal amplifier 224 to the level which is required for analog-to-digital conversion. The temperature signal thus amplified is applied to the analog-to-digital (A/D) converter 225, where it is converted into a digital temperature signal of several bits. The digital temperature signal is supplied to the microcomputer 226. The microcomputer 226, receiving a control signal provided by the print control means 111 for every line, changes the digital temperature signal for every line which is always delivered from the analog-to-digital converter and is variable.
The reason why the temperature signal is changed for every line as described above is as follows: Although the head temperature increases during the printing operation, the temperature signal cannot be changed during the transfer of data. Therefore, it goes without saying that it may be changed for every gradation.
The joint data supplied to the data ROMs 109 are corrected into joint correction data, respectively, which are applied to the temperature characteristic correction data selector 222. Of the joint correction data with different temperatures which are transferred to the temperature characteristic correction data selector 222, one which is suitable for the temperature at that time instant is selected by the digital temperature signal which changes every line, and is stored in the correction data memory 110.
Next, the printing data are supplied to correction data inserting circuit 112 by the line memory 108. In this operation, the data on the joint portion, after being replaced by the correction data stored in the correction data memory 110, is applied to the comparator 218 in the data processing circuit 115, where it is compared with the gradation data provided by the gradation control means 227. The data processing circuit 115 divides the data into data (1) and data (2) which are transferred to the blocks A and B of the head, respectively. In this operation, the white data provided by the white data generating circuit 114 is inserted alternately into the data (1) and the data (2). The white data is inserted into the data (1) and the data (2) for every line, and the printing of one line is accomplished by printing the block B and the block A. On the other hand, the gradation pulse generating means 116, which is controlled by the gradation data outputted by the gradation control means 227, applies a gradation data parameter to the head.
The conventional video printer is designed as described above. Therefore, the video printer is disadvantageous in that the white stripe attributed to the block division type drive of the thermal head, which is intended to use electric power more economically, can be corrected, but cannot be eliminated. In addition, the conventional video printer suffers from technical problems in that it has no aperture correction means which should be provided for a video printer, or it has no high precision color conversion means which is essential for fine copy images.